MOTM

On Friday 09 January 2004 13:01, Richard Brewster wrote:
> Here is a simple explanation:  DC (direct curent) means a
> steady, unchanging voltage, which could be any voltage
> level.  If a circuit is DC-coupled it means that the
> level of an unchanging voltage at the input will be
> passed through.  If a circuit is AC coupled then only
> changing voltages will be passed and there will be a
> "high pass" function applied, i.e. there will be a
> frequency below which signals won't pass. This cutoff
> frequency is normally about 20 Hz  for audio processing. 
> If you look at a circuit diagram, you will see a
> capacitor in series with the signal path if it is AC
> coupled.  Both inputs and outputs can be AC coupled.

I see. Is there also a low pass function applied on AC 
coupled circuits, at say 20 kHz?

> If you feed a DC voltage, say +5V, into an AC coupled
> input, the output will be zero.  You can always feed an
> AC signal into a DC coupled input.  

Ah, this helps! So I can go anywhere on my modular with an 
AC signal, on both AC and DC coupled inputs. Is that 
because a DC coupled input 'sees' the AC signal as a 
quickly changing series of 'unchanging voltage levels'?

> That works fine.  So why not always just use DC coupling?
>
> The benefit of AC coupling is that it removes any DC
> offset (constant) voltage.  For example, the signal input
> of a VCA is sometimes AC coupled, because a DC offset at
> the signal input can result in a popping sound when it is
> multiplied with the control input.

Okay.

>  The control input of
> a VCA has to be DC coupled, or else a prolonged
> unchanging voltage, such as the sustain portion of an
> envelope, would not produce the desired constant output
> level.

Understood.

Thanks Richard, that helped a lot!
- Robert